Your original post was months ago, but I wanted to add my 2 cents (better late than never.....)
The duplication issue is a result of the non-uninstallable HTC Sense "feature" that is preloaded on their phones. The only solution that I've found to delete all of the duplicate calendar entries is to:
1) Calendar>More>Calendars, deselect the Facebook calendar.
2) Settings>Accounts, remove the "Facebook for HTC" account (or Facebook for Touch on a myTouch 4g)
3) Reboot the phone
4) Go back to settings and add the Facebook account again.
5) Calendar>More and select the Facebook checkbox.
The reboot step is not entirely necessary, but I do it anyway.